142 GEOLOGY. 
that such exist, we shall have a key by which to interpret the rock-forma- 
tions of the past. Let us consider, therefore, the Rock-forming Agents. 
Volcanic Agents——The most obvious rock-formers at present in 
action are volcanoes. From circular openings, called craters! from 
their cup-like shape, at the summits of these mountains, there issue 
forth at certain times great streams of molten lava, boiling water, 
red-hot fragments of rock, mingled with flames, and smoke, and steam, 
amidst confused and thundering sounds, and the general convulsion 
of the surrounding country. These lava-streams, increased by ashes 
and other substances, are often of great thickness, sufficient to bury 
cities ; as Vesuvius once did Herculaneum and Pompeii, and Etna did 
Catania at its base, where the river of lava gradually rose round the 
walls, finally drowning the city in its burning flood, after it had flowed 
twenty-four miles! Successive accumulations of such outbursts deposit 
immense masses of rock, in the course of ages, round the centre of eruption ; 
so great, indeed, that the larger portion of such mountains—and some of 
those in America are five miles in height—are formed of the successive 
accumulations of the crater itself. The molten lava assumes various 
appearances after it has lost its heat: under water, it remains hard and 
compact ; in the open air, it becomes porous and cindery ; and in certain 
cases, it assumes a columnar form. AIl around, lie light pumice-stone, slag- 
like masses, fine pulverised dust, and huge calcined blocks. Now, the 
Unstratified rocks resemble in every feature these volcanic discharges. 
We meet with the compact lava in our trap and greenstone ; with the 
cinder, in the lighter porous rocks; with the ash, in our trap-tuffs ; and 
with the columnar, in the basalt. In exposed sections, we see the very 
channel through which these masses burst and overflowed the strata 
above ; and can trace the boundaries of the ancient molten streams in the 
cliffs and hills that everywhere vary the surface of the country. We can 
also see hardening and crystallising changes produced on the surrounding 
strata, wherever the heat of the erupted matter penetrated. We have 
therefore found the explanation of one great class of the rock-formations, 
the Unstratified, in the volcanoes scattered over the globe, that are at 
this moment depositing masses identical in kind with those that issued 
from the bowels of the earth in bygone ages. Such rocks, therefore, are 
termed zgneous,* from being produced by fire; volcanic, from having issued 
from volcanoes ; and eruptive, from being produced by eruptions. 
Aqueous Agents.—Rivers, as they flow over their channels, gather 
accumulations of mud, sand, gravel, and animal and vegetable remains, 
according to the size of the stream and the character of the country 
through which they pass; and these they deposit at their mouths in 
seas or lakes. Sometimes the amount of débris thus deposited is 
/ 
. 1 Greek, ‘a cup,’ 2 From Latin ignis, fire. 

